A conventional optical disk apparatus and the reproduction of a signal recorded on a disc will be described with reference to the accompanying drawings.
In the conventional optical disk apparatus as shown in FIG. 1, the light emitted from a semiconductor laser 8 is made parallel light by a condenser 9 and passed through a half-mirror 30 and through a focus lens 31 to be focused on a reflecting surface 16 of an optical disk on which signal marks are formed. Part of the incident light to the reflecting surface is reflected from the reflecting surface 16, and the reflected light which includes the information of the signal recorded on the disc is conducted again through the focus lens 31 to the half-mirror 30 and reflected therefrom into a light detector 32, where the recorded signal on the disc is detected. This detected signal is amplified by an amplifier 33.
The density of the recorded signal which can be reproduced by this conventional optical disc apparatus is limited as will be mentioned below.
The disc has a large number of very small signal marks recorded thereon These signal marks can be formed in various modes; for example, very small regions on the recording film are heated to change the phase of the film material and thereby to change an optical constant of the very small regions, very small pits or bumps are formed on the disc, and so on.
The information recorded in these signal marks can be reproduced by irradiating a very small spot of focused laser light on the signal marks and receiving the reflected light therefrom.
In the optical disc apparatus having the arrangement shown in FIG. 1, when the wavelength of the laser 8 is .lambda.=780 nm, the numerical aperture of the focus lens 31 is NA=0.50, and the intensity of the laser is constant within the aperture, then the light intensity distribution I (.xi.,.eta.) of the light irradiated on the reflecting surface ((.xi.,.eta.) coordinates system) of the disc is indicated in FIG. 2 by a curve (a). Here, if u (.xi.,.eta.) is the light amplitude distribution, the light intensity distribution can be expressed as I(.xi.,.eta.)=.vertline.u(.xi.,.eta.).vertline..sup.2.
In the following consideration of reflection from the reflecting surface of the disc, it is assumed that a very small and single encircled mark D is formed on the reflecting surface of the disc and the coordinates of the center, the amplitude reflectivity ratio and the phase difference of the signal mark are represented by (.xi..sub.3, .eta..sub.c), A.sub.s and .phi..sub.s, respectively.
The reflection function is given by the following equation: ##EQU2## The above equation can be rearranged by use of the delta function .delta. as ##EQU3## EQU where, A.sub.R =A.sub.s exp (i.phi..sub.s)-1 (3)
The integration term of the equation (2) is rewritten in a discrete form as in the equation (4): ##EQU4## where D is the area of the signal mark D, and n is the number of a group of points (.xi..sub.m,.eta..sub.m) and large enough to cover most of the region D.
When the signal mark D is narrower than the focused beam spot (for example, the diameter ratio is about 1/2 or below), u(.xi..sub.m,.eta..sub.m) (m=1, 2, . . . , n) can be represented by the value at the center (.xi..sub.c,.eta..sub.c) of the signal mark D.
Moreover, the aperture is assumed to be in the range of .alpha..ltoreq..theta..beta. (.theta. is the angle between the optical axis and the light ray) as viewed from the focal point F. Here, if .alpha.=0, the aperture is circular, and if .alpha.&gt;0, the aperture is of a ring band.
The amount of reflected light detected by the light detector (or focused by the focus lens) is given by the following equation: EQU T(A.sub.R,.xi..sub.c .eta..sub.c)=T.sub..phi. +D.sub.s (A.sub.R +A.sub.R.sup.*)I( .xi..sub.c,.eta..sub.c)+(2.lambda.I.sub.-1 D.sub.S.sup.2 /.pi..sup.2).vertline.A.sub.R .vertline..sup.2 I(.xi..sub.c,.eta..sub.c)(5)
where the sign * represents the conjugate complex number, and T.sub.100 and I.sub.n are respectively defined by the following equations: ##EQU5## Thus, the standardized reproduced signal amplitude .DELTA.P (the difference of the amounts of detected light corresponding to the presence and absence of the signal mark) is given as: ##EQU6##
In order to satisfactorily reproduce a signal from one signal mark and to prevent the so-called cross-talk, it is necessary to separate the other adjacent signal marks from the reproducing signal mark by a certain distance. If, for example, the wavelength is .lambda.=780 nm and the aperture is a circular aperture of NA=0.50, this distance is found as follows.
If, now, the center of the beam spot is at the coordinates (0, 0), and the signal mark is at the coordinates (.xi..sub.c .eta..sub.c) respectively, the ratio of the cross-talk signal to the detected signal can be expressed by EQU .DELTA.P(A.sub.R,.xi..sub.c,.eta..sub.c)/.DELTA.P(A.sub.R,0,0)=I(.xi..sub.c ,.eta..sub.c)/I(0,0) (9)
Thus, from the curve (a) in FIG. 2, it will be found that in order to reduce the cross talk to -30 dB or below, or to satisfy .DELTA.P(A.sub.R,.xi..sub.c,.eta..sub.c)/.DELTA.P(A.sub.R O,O).ltoreq.0.30 (.xi..sub.c.sup.2 +.eta..sub.c.sup.2).sup.1/2 .gtoreq.0.86 .mu.m must be satisfied. In other words, if the diameter of the signal mark is represented by d.sub.D, the distance between the adjacent marks must be (d.sub.D /2+0.86) .mu.m or above
Generally in the recording optical disc, the diameter of the signal mark is determined by the spot diameter d of the laser beam, or substantially d.sub.D .apprxeq.d/2. The general spot diameter d of the focused light (or the diameter of the first dark line ring of an Airy disc) is given as d=1.22 .lambda./NA.
Thus, when .lambda.=780 nm and NA=0.50, the track density up to 1.3 .mu.m and the line density up to 1.3 .mu.m are allowed as the limit of satisfactory reproduction of the signal, and hence these values are the upper limit of the actual signal density of the optical disc.
The idea that the signal density is increased by using the ring band shaped aperture for reducing the spot diameter d is not realistic. The reason for this is as follows. The curve (b) in FIG. 2 shows the light intensity distribution for the case in which the aperture is of a ring band satisfying sin .theta.=0.44 to 0.60. The diameter of the main lobe is decreased, but the peak value of the first side lobe is increased to about 14% of the peak of the main lobe. The signal density depends on the spot diameter d of the main lobe as described above, and thus it appears that the signal density can be increased by use of the ring band shaped aperture in which case the spot diameter d is decreased. However, in practice, the increase of the peak of the first side lobe causes the cross-talk to increase, thus deteriorating the signal reproduction ability. As a result, when the reproduction is considered, the signal density cannot be increased.
In addition, if the wavelength of the laser source 8 is decreased, and if the numerical aperture of the focus lens 31 is increased, the signal density can be increased without deteriorating the signal reproduction ability.
However, the decrease of the wavelength of the light source, particularly of the semiconductor laser, is extremely difficult. It will take a considerably long time to develop a large-power short-wavelength laser.
Moreover, as the numerical aperture of the focus lens is increased, (1) the focal depth becomes small and the allowance as to the errors in the tilt angle of the lens or the disc and in the defocus is narrower, and (2) the focus lens becomes large and heavy because the working distance cannot be decreased for preventing the contact between the disc base and the lens, thus deteriorating the frequency characteristics of the focus lens drive mechanism (pickup). Accordingly, the increase of the numerical aperture of the focus lens is limited.
Thus, in the conventional optical disc apparatus, it is practically impossible to reproduce a signal which is recorded at a high density exceeding the diffraction limit of light.